1. Field of the Invention
The present invention relates to a process for deactivating catalyst residues within polyolefin polymer particles in a post-reactor vessel by contacting the polymer particles with carbon dioxide (CO.sub.2) followed by melt extrusion or other means of incorporating an acid acceptor and at least one secondary antioxidant. The secondary antioxidant preferably is selected from organic phosphites, organic phosphonites, aromatic lactones or N,N-dialkylhydroxylamines and related species. The resulting product displays improved color and reduced melt flow breaking tendency.
2. Description of Related Art
Catalyst deactivation has been a pivotal part of polypropylene technology since its inception. Product discoloration and melt flow instability were hallmarks of inadequate deactivation, particularly with the old conventional Ziegler-Natta catalysts where product titanium levels where generally high. Indeed, an extraction step was often needed to reduce catalyst levels. With the advent of super high activity catalysts exemplified by U.S. Pat. Nos. 4,728,705, 5,034,361; 5,082,907; 5,151,399; 5,229,342; 5,106,806; 5,146,028; 5,066,737; and 5,077,357, levels of titanium in the product became so low that deactivation became a more manageable process, and the focus was as much on the elimination of reactive alkyl species as it was on the deactivation of titanium.
Many ways to deactivate polypropylene in association with product finishing have been envisioned over the years in association with a variety of patents relating to the use of alcohols, water based systems, and other approaches. Mention is made of some of these technologies in "Polypropylene and other Polyolefins", authored by Ser van der Ven (Elsevier, Amsterdam, p. 121(1990)). Deactivation with water or steam has advantages because of the high reactivity of water with the aluminum alkyls and with other catalyst related species as well. Further, it offers convenience and economy of use. However, the water may hydrate residual magnesium chloride within the powder to make the hexahydrate which later at melt compounding temperatures undergoes internal hydrolysis, releasing HCl. The thermal decomposition of the hexahydrate has been described by Petzold and Naumann (J. THERMAL ANALYSIS, 19, 25 (1980)); by. Dutt, et al (Indian J. Technol., 10, 41 (1972)); and by Heide and Eichhorn (J. THERMAL ANALYSIS, 7, 397 (1975)). In addition, with high rubber content impact copolymers, steam treatment by virtue of heat and moisture can make the rubber phase more cohesive, and thus potentially limits the upper rubber content (Fc) limit. An association between greater ease of emptying a product purging vessel with shutting off the steam treatment has been observed in connection with commercial unit production of impact polypropylene with a high Fc level. Reference to handling problems caused by excessive moisture in olefin polymers during the deactivation process can be found in Brod and Garner, U.S. Pat. No. 4,758,654.
In application to other polyolefins in general, steam may have the same adverse effects leading to particle agglomeration, particularly when inherent crystallinity is compromised by the presence of a comonomer phase. Further, it is advantageous to introduce steam near the discharge orifice from the deactivation vessel, with the steam directed upward as described by Bernier, U.S. Pat. No. 4,731,438. While this is desirable for reasons cited by Bernier and not an issue with relatively crystalline polyolefins, having hot steam in the vicinity of the discharge orifice creates a potential for particle agglomeration in that critical area when high comonomer content polyolefin polymers are manufactured. As is well known in the art, particle agglomeration can lead to bridging and ultimately plugging of the vessel orifice.
It is known that CO.sub.2 will react with aluminum alkyls to form carboxylated aluminum species as has been described by Sonntag and Zilch ("Synthetic Fatty Acids", in FATTY ACIDS AND THEIR INDUSTRIAL APPLICATIONS, Edited by E. S. Pattison, Marcel Dekker, NY, p. 365 (1968)). Mirviss and Inchalik, U.S. Pat. No. 2,827,458, disclose this reaction as a step in an approach to making carboxylic acids having an odd number of carbon atoms. Temperature and pressure play an important role in the reaction, however, and the use of gas-solid media has not been explored in prior art references. While the authors of the latter reference show three moles of CO.sub.2 reacting with one mole of aluminum trialkyl species, other work by Ziegler, et al (Liebigs Ann. Chem., 629, 251 (1960)) suggests that it is more likely that one or at most two moles of CO.sub.2 are incorporated. The latter was accomplished at high pressures (about 150 to 180 atm) and temperatures (220.degree. C. to 240.degree. C.).
Ishimoto et al (Patent Bureau of Japan, Official Gazette for Unexamined Patents, Disclosure No. S61-98707, May 17, 1986; Application No. S59-219396, Oct. 20, 1984) disclose the use of CO.sub.2 as a catalyst deactivator in slurry polymerization in an application concerning the use of "catalyst inactivators" selected from a group of oxygen containing compounds such as "CO, CO.sub.2, water alcohol, ether and ketone". The application is directed to the slurry polymerization (TiCl.sub.4 with TEAL, etc.) of ethylene or propylene copolymers of 1-olefins of at least 5 carbon atoms, and the inactivator is specifically intended for eliminating unreacted comonomer (C5 or larger) from undergoing post-reactor polymerization in a flash drum. When this type of polymerization occurs, the result is fish-eyes in the product. The CO.sub.2 treatment takes place during removal of monomers from the solvent-polymer slurry. This provides no added benefit, however, with respect to avoidance of polymer stickiness and associated agglomeration since a liquid medium is maintained during the treatment step. JP 61176611 A2, Aug. 8, 1986; DuPont Canada (Priority: GB 85-2067, Jan. 28, 1985), disclose the high temperature solution polymerization of ethylene or a mixture containing C4 to C12 olefins. Titanium or vanadium catalysts were used with DEAC. Deactivators included CO.sub.2 and CO among other species. The process is described as providing discoloration resistant polymer without removing residual catalysts. In an example, CO.sub.2 is used along with a cyclohexane solution of caprylic acid-calcium caprylate. Good color was obtained.
In slurry or solution polymerization, catalyst deactivation with CO.sub.2 offers no process advantage that can be attributed to a gas phase process, especially with respect to copolymers containing a rubber phase. In the presence of solvent or the typical hydrocarbon diluent, rubber phases from polypropylene impact copolymers and polyolefins with high comonomer content either dissolve or partially extract, leaving sticky coatings on the ultimately isolated polymer particles, thus eliminating any advantage CO.sub.2 might have offered in regard to reducing a tendency for particle agglomerations.
In other disclosures, Ohtani, et al., EP No. 829,491 A2, describe the use of agents (one of which is CO.sub.2) to quickly terminate gas phase olefin polymerizations through direct contacting of the agent and the reaction medium in the reactor. The purpose is to permit use of the reactor granules as seed powder for subsequent polymerization without treatment to remove the deactivator. In a similar mode, Kersting, et al., U.S. Pat. No. 5,344,885 teach mechanisms to deactivate a pre-activated Ziegler-Natta catalyst and then reactivate the catalyst before polymerization. Further, Kersting, et al. report that this improves reactor product morphology in terms of fewer fines and fewer coarse particles. In addition, WO 98/30599 disclose temporarily idling a polymerization reaction by injecting a sufficient amount of a kill gas that may include CO.sub.2, and then subsequently restarting the polymerization reaction by adding additional catalyst. Because the kill gas is introduced into the reaction zone, catalyst activity and thus production efficiency is compromised, and vent gases must be removed from the reactor. Thus the method would not be effective or efficient for deactivating catalyst residue during continuous production in the gas phase reactor.
Alcohols and carboxylate species are known to inhibit, rather than promote Ziegler-Natta polymerization as has been disclosed by van der Ven as referenced above. According to Ziegler, et al in their above cited reference, the autooxidation of alkyl aluminum compounds with subsequent hydrolysis proceeds as follows: EQU R.sub.3 Al+1.5 O.sub.2 .fwdarw.(RO).sub.3 Al EQU (RO).sub.3 Al+3 H.sub.2 O.fwdarw.Al(OH).sub.3 +3 ROH
The alkoxide is formed via the intermediacy of peroxidic species, as is known in the art (cf A. G. Davies and C. D. Hall, J. Chem. Soc., 1192 (1963)). While most of the peroxidic material is converted to alkoxide, some may remain (Davies and Hall) to later cause discoloration through reaction to produce chromophores.
When water or steam is used as the deactivator, the pertinent reaction with aluminum alkyls is well known in the art, and is given as follows: EQU R.sub.3 Al+3 H.sub.2 O.fwdarw.3 RH+Al(OH).sub.3
As mentioned above, Bernier, U.S. Pat. No. 4,731,438, and Brod and Garner, U.S. Pat. No. 4,758,654, disclose technologies associated with catalyst deactivation with aqueous media. In accordance with the above reaction of aluminum alkyls with water or steam, volatile alkenes may be produced in a quantity that can be substantial with commercial unit production over time. Production of volatile alkenes can be extremely hazardous, especially in subsequent high temperature material handling or processing steps downstream from the reactor environment.
It has not been known previously to deactivate catalyst residue using CO.sub.2 within an extruder. Hughes, et al., U.S. Pat. No. 5,756,659, describe means to improve the oxidative thermal stability of ethylene polymers by removing residual unreacted monomer, solvent and thermally unstable species using inert stripping agents, including CO.sub.2, water and steam, introduced into at least one stripping agent port. Removal of volatile species is effected in at least one vacuum zone downstream from the point or points of stripping agent introduction. Hughes, et al. is not concerned with catalyst residue deactivation, and does not suggest the use of CO.sub.2 as an active species that can react with unreacted catalyst residue present in a polymer product.